Dextromethorphan is an antitussive used in many over-the-counter cold medications. It has an opioid-like structure but, being a d-isomer, it does not possess the analgesic or addicveti retes of opioids. It acts centrally to relieve cough, similarly as opioids. It is active against dry cough and does not exhibit significant expectorant properties for productive cough.
As is well known, the maximum time of effectiveness in dextromethorphan compositions is only a few hours because of biological modification and/or elimination of the medication in the body (i.e., the usual doses for immediate-release formulations range from 15-30 mg every 4-6 hours). Consequently, repeated dosages must be taken at frequent intervals to obtain long term therapeutic levels of drug. Furthermore, this drug usually dissolves readily in the digestive juices and the total dosage is immediately fed into the blood stream. After high initial peak concentrations, the level of drug in the blood stream constantly decreases because of the biological elimination, so there is little or no therapeutic effect at the end of the period between dosages. As a result, the therapeutic effect fluctuates between dosages corresponding to the peaks and valleys in the level of drug in the blood as commonly measured by trough to peak ratios.
It is possible to incorporate materials into dextromethorphan compositions in such a manner that the dextromethorphan is liberated from the compositions at a predetermined rate. In this way, it is possible, for example, to achieve a prolongation of the period of action and thus to avoid too quick and/or too concentrated a release of the dextromethorphan and too high peaks of the blood or tissue levels, which can lead to undesirable side effects. DELSYM DM, marketed by Ciba Pharmaceuticals, is an example of a sustained-release dextromethorphan composition. The sustained-release characteristics of the composition are achieved by use of small particles of an ion-exchange resin bound to the dextromethorphan which delay release of the drug in the gastrointestinal tract.
While sustained-release of dextromethorphan is achieved by compositions such as DELSYM DM, certain advances to this area of technology are required. The bioavailability of dextromethorphan from these type of compositions is relatively low due to the strong bond between residual amounts of dextromethorphan and the ion exchange resin. The total percentage of dextromethorphan released from dextromethorphan/ion exchange resin complexes is incomplete, and under certain in vitro conditions it has been observed that even after a twelve hour period of time, about 20% of dextromethorphan remains bound to the ion exchange resin.
It is an object of the present invention to provide a sustained-release antitussive liquid formulation.
A further object of the present invention is to provide a sustained-release antitussive liquid formulation which is not dependent on the presence of large amounts of drug/resin complexes.
Another object of the present invention is to provide a sustained-release antitussive liquid formulation which is capable of demonstrating a release profile which will result in up to a twelve hour cough relief